Chinese stocks have been on a tear over the last year. Mainland stocks are up 25% just this year. They were probably due for a pullback and they got one on Friday. Chinese authorities tightened rules on margin selling (and made it easier to do short selling). But they did it after the Chinese stock market had closed. That explains why the Shanghai market showed gains on Friday. The selling was reflected in Chinese ETFs which trade all day. Chart 1 shows the CSI 300 China A-Shares Fund (which tracks stocks in Shanghai and Shenzen) tumbling 5% in heavy trading.